1. Field of the Invention
The present invention relates to an inkjet printing apparatus, a printing control method for the inkjet printing apparatus, a program, and a storage medium and, more particularly, to an inkjet printing apparatus which prints by reciprocally scanning an inkjet printhead and performing reciprocal printing on a printing medium such as printing paper, a printing control method for the inkjet printing apparatus, a program, and a storage medium.
2. Description of the Related Art
A printing apparatus is used as a printing unit for images in a printer, a copying machine, or a facsimile apparatus, or as an output device for a multi-functional peripheral including a computer, a word processor, a workstation, or the like. This printing apparatus is configured to print image information such as characters and images on a printing material (to be also referred to as a printing medium hereinafter) such as printing paper, a thin plastic plate, or the like on the basis of image information (including all output information such as character information).
Such printing apparatuses can be classified into an inkjet system, wire dot system, thermal system, laser beam system, and the like according to the printing method. Of these systems a printing apparatus based on the inkjet system (to be referred to as an inkjet printing apparatus hereinafter) prints by discharging ink from a printing unit including a printhead onto a printing medium. This system therefore have various advantages as compared with the remaining printing systems, for example, higher resolution, higher speed, quietness, and lower cost. On the other hand, recently, with an increase in the importance of color outputs such as color images, many color inkjet printing apparatuses with high image quality equivalent to silver halide photographs have been developed and are commercially available.
Such an inkjet printing apparatus uses a printhead obtained by integrating a plurality of ink discharge apertures and fluid channels as a printhead on which a plurality of printing elements are integrated and arrayed to increase the printing speed. In addition, in order to cope with color printing, an inkjet printing apparatus is generally used, which comprises a plurality of printheads each obtained by integrating a plurality of ink discharge apertures and fluid channels.
Although various systems are known as printing systems for printers, the inkjet system, in particular, has attracted attention because, for example, it can perform noncontact printing on a printing medium such as printing paper, easily allows for color printing, and is very quiet in quietness. In general, a serial printing system is widely used because it has low cost and is compact. This system uses a printhead which discharges ink in accordance with printing information to be printed and performs printing while reciprocally scanning the printhead in a direction perpendicular to the feeding direction of a printing medium such as printing paper.
Recently, such inkjet printing apparatuses have greatly improved in performance, and have been achieving high printing speeds almost as high as those of laser beam printers. In addition, with an increase in the processing speed of personal computers and the proliferation of the Internet, demand has arisen for an increase in printing speed for color images.
Furthermore, there has recently been strong demand for an improvement in the quality of printed images in addition to an increase in printing speed. In order to simultaneously satisfy these requirements, there has been developed a printing apparatus which can print images with high resolution by using a printhead which has achieved a further reduction in ink droplet size and an increase in nozzle density.
In addition, with the demand for an improvement in the image quality of an inkjet printing apparatus, a printhead having nozzles capable of discharging droplets in different ink discharge amounts has been under development in order to perform tone representation with higher resolution. There has already been commercially available an inkjet printing apparatus which can perform smooth tone representation by reproducing ink droplet sizes of different dot sizes (for example, large and small dots) on printing paper.
Of the inkjet systems, an air bubble jet printing system (BJ system) is designed to discharge ink droplets from orifices with the pressure of air bubbles generated by abruptly heating and vaporizing ink using heating elements. The air bubbles generated by the printhead of the bubble jet system having this structure are cooled by the surrounding ink, and ink vapor in the air bubbles condenses and returns to the liquid. As a consequence, the air bubbles disappear. On the other hand, the ink consumed by discharging is replenished from ink storing in ink tanks through ink supply paths.
FIG. 1A is a view showing an example of the arrangement of the nozzles (discharge apertures) of an inkjet printhead 101, which has a plurality of nozzle arrays 102 so as to discharge different kinds of inks. As shown in FIG. 1B, each nozzle array 102 is configured such that a discharge aperture array 103L with a large amount of ink discharge is placed on the left side of an ink supply path 105, and a discharge aperture array 103S with a small amount of ink discharge is placed on the right side of the ink supply path 105. Using the respective discharge aperture arrays makes it possible to print large and small dots on printing paper. Ink is supplied from the ink supply path 105 through ink flow paths 104 provided in correspondence with the nozzle arrays 103L and 103S.
The printhead 101 shown in FIG. 1A is used for color printing, and is configured such that two pairs of nozzle arrays, each having a combination of nozzles with large and small ink discharge amounts, are respectively arranged on the left and right sides (a total of four arrays for two colors), and two nozzle arrays, each having a combination of nozzles with the same ink discharge amount, are arranged in the middle. With this arrangement, the printhead 101 can perform color printing by symmetrically discharging cyan (C) ink and magenta (M) ink and discharging yellow (Y) ink and black (BK) ink in the middle.
FIG. 2 shows the arrangement of the main part of the inkjet printing apparatus for printing on printing paper P by using the inkjet printhead 101 shown in FIGS. 1A and 1B (this operation will also be referred to as printing hereinafter) Referring to FIG. 2, reference numeral 201 denotes an inkjet cartridge (printing unit). These inkjet cartridges 201 comprise ink tanks for ink in four colors and printheads. That is, the inkjet cartridges 201 comprise ink tanks respectively storing black (BK), cyan (C), magenta (M), and yellow (Y) inks and the printheads 101 corresponding to the respective inks.
Reference numeral 203 denotes a paper feed roller (subs-scanning unit), which rotates in the direction indicated by the arrow in FIG. 2 while clamping the printing paper P together with an auxiliary roller 204, thereby intermittently conveying the printing paper P in the Y direction. Reference numeral 205 denotes a pair of feed rollers which feeds the printing paper P. The pair of feed rollers 205 rotates while clamping the printing paper P, like the rollers 203 and 204, but rotates at a rotational speed lower than that of the paper feed roller 203 to produce tension on the printing paper P. This makes it possible to convey the printing paper P without any flexure.
Reference numeral 206 denotes a carriage (main scanning unit) which reciprocally moves (or reciprocally scans) in the main scanning direction (the X direction in FIG. 2) perpendicular to the Y direction while supporting the four inkjet cartridges 201. The carriage 206 stands by at a home position h indicated by the broken line in FIG. 2 while the printhead 101 performs no printing operation or during a recovery process for the printhead 101.
Upon receiving a print start command, the carriage 206 located at the home position h before the start of printing performs printing with a width of N/1200 inches on the printing paper P by using N (N is an arbitrary positive integer) nozzles of the printhead 101 while scanning in the X direction in FIG. 2. Upon completion of printing to an end portion of the printing paper P, the carriage returns to the home position h, and scans again to print in the X direction. Before the start of the second printing operation after the completion of the first printing operation, the paper feed roller 203 rotates in the direction indicated by the arrow to feed the printing paper P in the Y direction by a width of N/1200 inches.
In this manner, the printhead 101 repeatedly performs printing operation with a width of N/1200 inches and paper feeding operation for every scanning by the carriage 206, thereby completing printing of an image signal corresponding to, for example, one page on the printing paper P. Note that the printing mode of completing printing within the same area by one scanning operation of the carriage 206 will be referred to as a 1-pass printing mode.
As another printing mode used for the inkjet printing apparatus, a multi-pass printing mode is available. The multi-pass printing mode is the printing mode of completing printing within the same printing area by overprinting a plurality of times in the printing area. It is generally known that in the multi-pass printing mode, a larger the number of passes results in a better image quality.
Note that FIG. 2 shows uni-directional scanning/printing operation of performing printing operation only at the time of forward scan of the carriage 206. However, when printing at a higher speed, the printing apparatus generally uses a bi-directional scanning/printing system of printing both at the time of forward scan and at the time of backward scan of the carriage 206.
It is known that when images are to be formed by using the inkjet printing system, continuously applying identical driving pulses to heating elements will cause density irregularity due to a rise in the temperature of the head which is caused by continuous heating by heaters. This density irregularity will be described in detail later.
For this reason, when performing high-image-quality printing in the inkjet printing system, it is important to reduce this density irregularity. As a method of reducing density irregularity, there is generally proposed a unit which uniformly controls discharge amounts or a unit which corrects printing data. Other known techniques of reducing density irregularity include a technique using the above multi-pass printing mode and a technique of setting the scanning speed to a relatively low speed although it has a demerit that the printing speed decreases.
The ink discharge amount of the printhead 101 depends on the temperature of the ink near each heating element when identical driving pulses are to be applied to the heating element. It is therefore necessary to manage the temperature of ink. It is, however, practically difficult to manage the temperature of ink, and hence a technique of controlling the ink discharge amount of the printhead 101 by managing the temperature of the printhead 101 instead of the temperature of ink is in widespread use.
For example, Japanese Patent Laid-Open No. 5-31905 discloses a technique of arranging a sensor for detecting the temperature of a printhead in the printhead and monitoring an output from the sensor by using an MPU (CPU) as a detection unit. There is also proposed a control method (PWM control) of shortening the heating time of a heater to suppress a rise in the temperature of the printhead by changing the pulse width of a pulse signal for heater driving when the temperature rises, thereby making the ink discharge amounts constant.
The sensor is, however, attached near the printhead, and hence the MPU (CPU) cannot monitor an accurate output due to noise caused by the driving of the printhead. This makes it impossible to perform accurate temperature control and to sufficiently control the ink discharge amount.
To cope with this problem, there is also a proposal of using a mechanism for amplifying a detected temperature output, a technique of taking countermeasures against noise in a detection result, and the like as well as providing the temperature sensor for the printhead. However, such a proposal leads to an increase in cost. For this reason, there has been proposed a technique of estimating the temperature of the printhead from image data to be printed in consideration of the reliability of the temperature sensor, and there has been a proposal using this technique in place of or together with the above technique.
For example, as a technique of estimating a temperature from printing data, there is known a technique of temporarily storing image data corresponding to one main scan in a memory area such as an image buffer before the execution of the next printing/scanning operation, and then estimating a head temperature from a count result obtained by counting the number of effective data in the image buffer, thereby modifying the driving condition for the head on the basis of the estimated temperature.
For example, Japanese Patent Laid-Open Nos. 5-208505 and 7-125216 disclose a technique of estimating a fluctuation in the temperature of an inkjet head from the amount of heat charged to the inkjet head per unit time, and modifying the width of a driving pulse on the basis of the estimation result.
Recently, with an increase in discharge frequency accompanying an increase in printing speed and with an increase in the number of nozzles per nozzle array, strong demand has arisen for the use of a technique higher in accuracy than the conventional temperature estimation method.
Shortening the time interval of temperature estimating calculation can achieve an increase in the accuracy of estimation of the temperature of the inkjet head. However, shortening the calculation time interval will increase the load of calculation on the main body of the printing apparatus. This will lead to the necessity of improving the performance of an MPU (CPU) as a calculating unit or a decrease in throughput.
In order to cope with this problem, Japanese Patent Laid-Open No. 7-125216 discloses, as a method of estimating a temperature with high accuracy and with a low calculation load from a driving condition for a printhead, a technique of estimating the temperature of the printhead and accurately controlling an ink discharge amount by executing the above PWM control in accordance with the estimated temperature.
More specifically, this technique converts the driving condition for the printhead into the amount of heat charged and accumulated in the printhead, and calculates the amount of heat accumulated after heat dissipation with the lapse of a unit time from the amount of heat accumulated in the inkjet head by the time of the previous printing/scanning operation. The technique then stores the amount of heat accumulated in the printhead for each thermal time constant, and adds each amount of heat charged to the amount of heat after heat dissipation, thereby calculating a rise in the temperature of the printhead.
Japanese Patent Laid-Open No. 8-156258 discloses a technique which is designed especially for large printing apparatuses and to estimate a temperature from image data in real time and perform discharge amount control. That is, this reference discloses a technique of counting effective data in image data, and when the count value reaches a predetermined value, modifying the width of a head driving pulse signal, or a technique of performing data correction by thinning printing data by a predetermined amount.
Consider an inkjet printing apparatus having a printhead including discharge apertures with different discharge amounts to print dots with different dot sizes on the printing paper P as described above. In this case, owing to demand for high image quality, even an apparatus with a discharge amount for a large size is required to discharge very small droplets in practice in consideration of image quality, more specifically, graininess. For example, an apparatus with about 5 pl as a large ink discharge amount and an apparatus with about 2 pl as a small ink discharge amount are on the market. The ink droplet sizes of these apparatuses are much smaller than those some years ago. In order to achieve a further improvement in image quality, it is required to further reduce the ink droplet size. When printing is to be performed by using such a printhead with a small ink discharge amount, the number of ink dots covering a printing area influences the size of the printing area.
When dots with a small ink discharge amount about half that in the case shown in FIG. 3A are to be used, the number of dots to be arranged to print in the same printing area with the same density needs to be twice in the vertical and horizontal directions, that is, four times that in the prior art as a whole, as shown in FIG. 3B. Obviously, if the number of nozzles and nozzle density of the printhead and the discharge frequency remain the same, the printing speed greatly decreases.
In order to maintain the printing speed almost equal to that in the prior art by using such a printhead with a small droplet size, there have proposed means for improving the performance of the printhead itself. These means include a method of increasing the printing width by increasing the number of nozzles and a method of increasing the discharge frequency of ink droplets. In addition, it is required to improve an image forming method by, for example, shortening the printing time in a printing area to increase the printing speed for each main scanning operation, and decreasing the number of passes in multi-pass printing operation.
Increasing the ink discharge frequency of a printhead will shorten the printing time required for one scan. This, however, increases the temperature of the printhead accompanying ink discharge during a scan and leads to an increase in ink discharge amount due to an operation error. In addition, an increase in the number of ink dots to be printed in a printing area owing to a reduction in droplet size will lead to a further rise in the temperature of the printhead and an increase in ink discharge amount errors. For this reason, the degree of rise in head temperature during one printing/scanning operation further increases. As a result, a further increase in ink discharge amount causes density irregularity in the printing area for each scan.
In addition, when high-speed printing is to be performed with the decreased number of printing passes in the multi-pass printing system, the number of ink dots to be discharged per scan increases with an increase in the number of passes. In this case as well, an increase in ink discharge amount owing to a rise in the temperature of the printhead will cause density irregularity in the printing area. Obviously, simultaneously increasing the ink discharge speed and the number of passes will have a very significant influence.
On the other hand, as the printing speed increases, the detection of a head temperature by a temperature sensor lacks in performance in terms of responsiveness. In addition, in detection by a unit which estimates a temperature from printing data, an increase in discharge frequency will decrease the maximum pulse width within one discharge timing. This narrows the controllable range of ink discharge amounts in the changeable range of pulse widths. That is, the control performance is not currently sufficient.
When a bi-directional printing system is used to print by high-speed ink discharging operation in the above 1-pass printing mode, the entire area printed for each scan has a density distribution in the main scanning direction. More specifically, as shown in FIG. 5, density irregularity occurs in the form of a band for each scan. This density irregularity is especially noticeable on the two end portions of the printing area.
FIGS. 6A to 6D are a view showing the state (FIG. 6A) of a printing area when a solid image with the same tone is printed by an arbitrary main scan in the above 1-pass printing mode, and graphs showing the relationship between the temperature of the inkjet printhead 101 and the ink discharge amount at that time.
Referring to FIGS. 6A to 6D, the head temperature rises in the process of printing operation in the main scanning direction of the printhead 101 as shown in FIG. 6B. With this rise in temperature, the ink discharge amount increases as shown in FIGS. 6C to 6D in both the case of nozzles (discharge apertures) with a small ink discharge amount and the case of nozzles (discharge apertures) with a large ink discharge amount. As a result, density irregularity occurs in the main scanning direction, as shown in FIG. 6A.
As described above, when a printhead comprises nozzles (discharge apertures) with different ink discharge amounts, the ink discharge amounts of the respective types of nozzles increase in different manners. In addition, if tone reproduction is performed with a combination of large dots formed by droplets with a large discharge amount and small dots formed by droplets with a small discharge amount, the density increases due to the influences of the respective ink discharge amounts.
In accordance with a rise in density in the process of printing/scanning operation, therefore, it is necessary to decrease the discharge amount of ink to print. For example, there is proposed a method of executing density correction in accordance with a rise in temperature while counting the dots of image data to be printed.
More specifically, the scanning area of the inkjet printhead 101 is divided into a plurality of count areas and printing areas in the scanning direction. This method then counts the number of dots printed in a count area and modifies image data in the next printing area in accordance with the counted number of dots, thereby forming an image. To modify the image data is to modify the data upon counting multi-tone image data, convert the modified multi-tone image data into binary data, and print it.
This method allows a decrease in the number of dots to be printed as the temperature of the printhead 101 rises and a reduction in density irregularity.
However, independently correcting the image data respectively formed by large dots and small dots discharged from nozzles (discharge apertures) with different ink discharge amounts will make the corrected positions overlap and will reduce the image data more than necessary. As a result, as shown on the right side of FIG. 4A, noticeably lightly printed portions “excessive bright portions” 401 appear only in the corresponding portions. That is, although the density irregularity is reduced, graininess may deteriorate.